Hydrogen evolution reaction catalyst

ABSTRACT

Systems and methods for a hydrogen evolution reaction catalyst are provided. Electrode material includes a plurality of clusters. The electrode exhibits bifunctionality with respect to the hydrogen evolution reaction. The electrode with clusters exhibits improved performance with respect to the intrinsic material of the electrode absent the clusters.

STATEMENT OF GOVERNMENT INTEREST

The United States Government has rights in the invention describedherein pursuant to Contract No. DE-AC02-06CH11357 between the UnitedStates Department of Energy and UChicago Argonne, LLC, as operator ofArgonne National Laboratory.

FIELD OF THE INVENTION

The present invention generally relates to water electrolysis catalysts.More specifically, embodiments of the present invention relate tocatalysts for use in the hydrogen evolution and oxygen evolutionreactions in an alkaline environment.

BACKGROUND OF THE INVENTION

Growing concerns about global warming and energy security demand theexpansion of renewable energy sources as viable alternatives tofossil-fuel-based technologies, in conjunction with improved energystorage options. In many of the innovative approaches to address thesechallenges, the production of hydrogen in various (photo)-electrolysissystems plays a pivotal role. Today, electrolytically produced hydrogencomes mainly from the chloralkali industry and water electrolysis. Inwater-alkali electrolysers (WAEs), for example, the cathodic-half cellreaction is the hydrogen evolution reaction (HER), the electrochemicaltransformation of water to molecular hydrogen and hydroxyl ions(2H₂O+2e⁻

H2+2OH⁻). The mechanism of the HER is typically treated as a combinationof three elementary steps: the Volmer step, water dissociation andformation of a reactive intermediate Had, (2H₂0+M+2e⁻

2M−H_(ad)+2OH⁻); followed by either the Heyrovsky step (H₂0+H_(ad)-M+e⁻

M+H₂) or the Tafel recombination step (2M−H_(ad)

2M+H₂). Adsorbed hydrogen species Had formed at potentials negative ofthe Nernst reversible potential for the HER is also referred to asoverpotentially deposited hydrogen (H_(opd)). The different states ofadsorbed hydrogen can also be referred, based on thermodynamicguidelines, as H_(upd)—the strongly adsorbed state and H_(ad)/H_(opd)—aweakly adsorbed state. Although the reactions pathways are similar, dueto the activated water dissociation step the HER activities for mostcatalysts in alkaline medium are usually ˜2-3 orders of magnitude lowerthan in acid solutions. The anodic-half cell reaction, the oxygenevolution reaction (OER), is a far more complex process, in which thehydroxyl ions generated at the cathode are consumed at the anode toproduce oxygen and water molecules (4OH⁻

O₂+2H₂O+4e−). Given the harsh conditions associated with the OER, thechoice of catalysts for electrolysis are typically noble metal oxidessuch as those of Ru, Ir and other forms of these. The poorconductivities and activities of the cheaper transition metal oxidessuch as that of 3d elements have limited their utilization in thesesystems. One way around it has been the use of high loadings of suchmaterials.

Given that the supply of water is virtually inexhaustible, the hydrogenand oxygen production in WAEs can, in principle, be highly economicaland almost limitless. In practice, however, large scale electrochemicalproduction of hydrogen from water splitting is greatly constrained bytwo fundamental limitations: (1) the high overpotentials (defined as thedifference between the reversible potential and the operating potential)of the HER and the OER in alkaline solutions, and (2) the lack ofstability of electrode materials. The HER and the OER play key roles ina wide range of areas, including water and chlor-alkali electrolysis,metal deposition, corrosion, and fuel production from CO₂ reduction. TheHER is also an electrochemical reaction of fundamental scientificimportance, since the basic laws of electrode kinetics, as well as manymodern concepts in electrocatalysis, were developed and verified byexamining the reaction mechanisms related to the charge-transfer-inducedconversion of protons (acid solutions) and water (alkaline solutions) tomolecular hydrogen.

It is not clear why the rate of the HER is ˜2 to 3 orders of magnitudelower at pH=13 than at pH=1, nor is it understood why the reaction issensitive to the catalyst surface structure in alkaline media butlargely insensitive in acids. A practical implication of the slowkinetics in alkaline solution is the lower energy efficiency for bothwater-alkali and chlor-alkali electrolyzers. For water-alkalielectrolyzers, the high overpotentials for the oxygen evolution reaction(OER) at the anode also contribute significantly to overall energylosses. This has led to various approaches to identify catalysts forboth OER and HER. However, rarely have these strategies for design ofmaterials been based on molecular level understanding of the reactionpathways. In addition, the influence of non-covalent (Van der Waalstype) interactions on the overall kinetics of the HER has been underexplored, particularly in light of recent studies highlighting theimpact of non-covalent interactions on the rates of many electrochemicalreactions such as oxygen reduction reaction, CO and methanol oxidationreaction.

Design and synthesis of materials for efficient electrochemicaltransformation of water to molecular hydrogen and of hydroxyl ions tooxygen in alkaline environments is of paramount importance in reducingenergy losses in water-alkali electrolysers. For decades, practicaldesign of metal catalysts for the HER in acidic media has been based onthe well-known concept of volcano plots. with rare exceptions, aclassical volcano-shaped correlation is found from both experimentalresults as well as computational approaches; with metals that adsorbhydrogen neither too strongly nor too weakly (the Pt-group metals)occupying the top of volcano. While the metals that adsorb hydrogen toostrongly (Ru, 3d-elements) are positioned on the descending part of thevolcano, the IB group metals which exhibit a weak M−Had interaction onthe ascending part. Similar plots also have been generated for the OERcatalyst materials; for simple oxides, RuO₂ and IrO₂ exist at the apexof the volcano, with other transition metal oxides in both the ascendingand descending portion of the curves. For more complex oxides, such asperovskites, similar positions exist with the metals in the ‘B’ site ofthe lattice determining the overall position in the volcano plot. Oneissue with the use of such volcano plots is the lack of clearinformation on active sites; for the theoretical calculation effort,ideal surfaces are used which seldom exist in reality. Similarly forexperimentally derived, rarely are the materials well-defined, whichresults in several ambiguities due to the contributions from otherfactors such as defects, inhomogeneities etc. . . . .

A great many materials have been tested for the HER and the OER inalkaline environments, including various combinations of metals, metalalloys, simple oxides such as RuO₂/IrO₂ (refs 15,16), and more complexmaterials such as combinations of 3d oxides, sulphides, phosphates andperovskites. Currently, various combinations of metals (pt, Pd, Ir, Ru,Ag, Ni), metal alloys (Ni—Co, Ni—Mn, Ni—Mo), metal oxides (Ru02), andNi-sulfides/Ni-phosphides are used to catalyze the conversion of H₂0 toH₂. Unfortunately, no current catalysts provide sufficient activity forhydrogen production (which is usually overcome with higher loading ofthese materials), thereby resulting in high overpotentials and energylosses. While most of the Pt group metals are good catalysts for theadsorption/recombination of the reactive hydrogen intermediates(H_(ad)), they are generally inefficient for the process of waterdissociation. On the other hand, although metal oxides (and in somecases other compounds such as sulfides) are effective for cleaving theH—OH bond, they are highly ineffective in converting the resultingH_(ad) intermediates to H₂. In addition, there are inherent issues withnon-noble materials stemming from the decrease in activity duringoperation, arising from the formation of hydrides as well as the overalldurability issues stemming from the dissolution of the catalystmaterials during intermittent start-stop operations. Some of theseissues have been overcome with alloying, in very high loadings of suchcatalyst materials (˜25-40 times the equivalent for Pt) in order toachieve the desirable activity. Similarly for the OER, given the harshconditions, the stability of the materials is critical. Given therelatively low stability of most of these materials, the norm of usinghigher loading is common. Also, the limitation with development of newcatalysts for the OER is the lack of clear fundamental knowledgerequired to design new catalysts.

Although these materials have shown interesting variations in catalyticbehavior from one catalyst to the next, all of the currently usedcatalysts operate at high overpotentials. One of the major reasons forthe slow progress in finding improved catalysts in WAEs is that theselection of these materials has been guided by a purely trial-and-errorand/or a combinatorial approach, and no studies focusing on a systematicunderstanding of trends in the fundamental, atomic-scale catalyticproperties of these reactions on well-characterized materials have beenestablished. Current state of the art materials including oxides andmetal catalysts are seldom cost effective, with noble metals having highmaterials cost and oxides having high performance cost.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to an electrode for use in thehydrogen evolution reaction. The electrode comprises an electrode metal.A plurality of metal (hydr)oxide clusters are deposited on the surfaceof the electrode metal. The electrode exhibits bifunctionality withrespect to the hydrogen evolution reaction.

One embodiment of the invention relates to a electrolytic cellcomprising an anode and a cathode. The cathode has a plurality of metal(hydr)oxide clusters deposited on the surface of the electrode metal.The electrolytic cell further comprises an electrolyte. The electrodeexhibits bifunctionality with respect to the hydrogen evolutionreaction.

One embodiment of the invention relates to a method of generatinghydrogen. A cell is formed having a cathode, an anode, and an alkalineelectrolyte. The cathode has a plurality of metal (hydr)oxide clustersdeposited thereon. A current is applied to the cell. Disassociation ofwater and the production of hydrogen intermediates is facilitated at theplurality of metal (hydr)oxide clusters. Hydrogen intermediates areadsorbed to the cathode surface. Hydrogen intermediates are combined toform molecular hydrogen.

Additional features, advantages, and embodiments of the presentdisclosure may be set forth from consideration of the following detaileddescription, drawings, and claims Moreover, it is to be understood thatboth the foregoing summary of the present disclosure and the followingdetailed description are exemplary and intended to provide furtherexplanation without further limiting the scope of the present disclosureclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe disclosure will become more apparent and better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1A illustrates sample XAS spectra for Co^(2+δ)O^(δ)(OH)_(2-δ) onPt(111) surface for two different potentials E=−0.1 V and E=+1.4 V. FIG.1B is a STM for Co(OH)₂/Pt(111) in the HER region. FIG. 1C is a STM forCoOOH/Pt(111) and includes the polarization curves for the OER. FIG. 1Dis a comparison of OH_(ad) charge as a function of oxophillicity of themetal oxide cation (M) for same coverages of theM^(2+δ)O^(δ)(OH)₂₋₈/Pt(111) surface.

FIG. 2 is a chart of the elements arranged in the order of theiroxophillicity from Mn to Ni as estimated for the well-defined systemshown in FIG. 1A.

FIG. 3 is a chart illustrating the trend in overpotential for the oxygenevolution reaction (OER) is shown as a function of the 3d transitionelements.

FIG. 4 illustrates the trend in overpotential for the HER for themetal-metal oxide system, where the elements are arranged in order oftheir oxophillicity from Mn to Ni. Pt is shown in the figure as areference.

FIG. 5A illustrates a graph of HER activity as a function of threedifferent coverages of Co(OH)₂% (bare Pt(111) surface), 40% (low) and62% (high); FIG. 5B illustrates a graph of OER activity as a function ofthree different coverages of CoOOH: 0% (bare Pt(111) surface), 34% and65% coverage on the Pt(111) surface. FIGS. 5A-B illustrate thefundamental difference between the HER and the OER namely: HER isbi-functional reaction (requires two types of sites) and OER ismonofunctional (requires one type of site)

FIGS. 6A-6C are STM images (60 nm by 60 nm) and CV traces for (A)Pt(111), (B) Pt(111) with 2D Pt islands, and (C) Pt(111) modified with3D Ni(OH)₂ clusters in 0.1 M KOH electrolyte. FIG. 6D is a graph of HERactivities for Pt(111), Pt-islands/Pt(111), and Ni(OH)₂/Pt(111)electrodes in acid solutions (a′ and b′) are shown to emphasize largeinitial differences between kinetics of the HER in alkaline versus acidsolutions.

FIG. 7A is an STM image (60 nm by 60 nm) and CV trace of theNi(OH)₂/Pt-islands/Pt(111) surface. FIG. 7B is a graphical comparison ofHER activities with Pt (111) as the substrate. The Pt-islands are usedto nucleate the growth of oxides to enhance the size distribution of theoxides.

FIG. 8 is a schematic representation of water dissociation, formation ofM−H_(ad) intermediates, and subsequent recombination of two H_(ad) atomsto form H₂ (magenta arrow) as well as OH⁻ desorption from the Ni(OH)₂domains (red arrows) followed by adsorption of another water molecule onthe same site (blue arrows).

FIG. 9A is a STM image (50 nm by 50 nm) and CV trace forNi(OH)2/Pt-islands/P(110) single-crystal surface. STM image showsNi(Oh)₂ randomly distributed on inherently rough Pt(110). FIG. 9B is acomparison of HER activities with Pt(110) as the substrate. FIG. 9C is atransmission electron micrograph image (50 nm by 50 nm) andcorresponding DV trace of Ni(OH)₂-free Pt-nano catalysts (TKK) with anaverage particle size of 5 nm. FIG. 9D is a comparison of HER activitieswith commercial nanocatalyst Pt/C (TKK) as the substrate. Incrementalimprovements in activities for the HER in 0.1 M KOH from unmodified Pt/Care shown for the hierarchical materials [surface covered with Ni(OH)₂]as the double layer (addition of Li⁺ cations). The activity for theunmodified Pt/C surface in 0.1 M HCIO₄ is shown for reference. Dashedarrow shows the activity trend. All the current densities for the TKKcatalyst system are normalized by the geometric surface area of theglassy carbon substrate. In one implementation, the system depicted inFIGS. 9A-B was found to have the best activities for the HER in alkalinesolutions to date.

FIG. 10 is a comparison between activities for the HER expressed asoverpotential required for 5 mA/cm² current densities, in 0.1 M HCIO₄and 0.1 M KOH for both bare metal surfaces and Ni(OH)₂ modifiedsurfaces. Metals are grouped into three distinct groups, IB group (Cu,Ag, Au), the Pt-group metals (Pt, Ir, Ru) and the 3d-transition elements(V, Ti, Ni).

FIG. 11 is a chart of activity enhancement achieved by the introductionof bi-functionality via Ni(OH)₂ addition to the surface of metal M. Ni,Ag and Cu are shown here for comparison. Ni for being the most active,while Ag and Cu offer unique advantage of low cost and low affinitytoward formation of hydrides, which has been known to decreaseactivities for the HER. Activities were measured from the firstpolarization scan, and the current densities at η=0.3V are plotted.

FIG. 12A illustrates a graph of activities for the HER, measuredexperimentally, plotted in the conventional volcano form for both acidand alkaline media. The activities exhibit a “volcano” trend, whichmatch what has been reported in the literature for acid solutions. FIG.12B illustrates cyclic voltammograms of bare substrate Pt, Ir, and Ru,as well as substrate modified with Ni(OH)₂. The variation inpseudocapacitance for the individual metals is clear from the scale barsfor the current densities. The relative coverages of Ni(OH)₂ is hard todetermine due to the lack of clear H_(upd) regions for more oxophilicmaterials (Ru, Ir). The Pt surface modified with Ni(OH)₂ was found to be˜12% covered for the CV shown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, and designed in awide variety of different configurations, all of which are explicitlycontemplated and made part of this disclosure.

HER catalyst activity varies depending on the pH of the Environment.Whereas in acid solutions the reaction is controlled mainly by thehydrogen recombination (the Tafel step), in alkaline solutions thekinetics are determined by a delicate balance between the waterdissociation (the Volmer step) and concomitant interaction of waterdissociation products with a surface. In one implementation, the presentinvention relates to systems and methods of tailoring a controlled andwell characterized arrangement of metal (hydr)oxide clusters onelectrode surfaces, such that an 2-8-fold increase in the activity ofthe HER relative to state-of-the-art metal and metal-oxide catalysts canbe realized. The catalytic proficiencies of metals and metal oxides arecombined by creating a bi-functional metal-oxide/metal material (metaloxides deposited on metal substrates) for the HER. In a bi-functionaleffect, the edges of the metal hydr(oxide) clusters promote thedissociation of water and the production of hydrogen intermediates thatthen adsorb (H_(ad)) on the nearby electrode surfaces and recombine intomolecular hydrogen. The composite materials of certain embodimentsfacilitate different parts of the overall multistep HER process inalkaline environments: an oxide to provide the active sites fordissociation of water, and a metal to facilitate adsorption of theatomic hydrogen produced and its subsequent association to form H₂ fromthese intermediates. This bi-functionality within the catalyst materialovercomes an otherwise a significant bottleneck in design activecatalysts.

In one embodiment, a material capable of disassociating water isdeposited on the surface of an electrode material. In one embodiment,the disassociative material is a metal (hydr)oxide. Preferably, thematerials is a 3-d transition metal (hydr)oxide, such as Ni(OH)₂. Facilewater dissociation properties of Ni(hydroxy)oxides compared to othertransition metal oxides have been well established but it should beappreciated that other materials for facilitating water dissociation maybe utilized. Other materials such as other transition metal oxides, aswell as any other metal-alloy oxides can also be employed. The range ofelements include, but are not limited to, Ni, Co, Fe, Mn, Pb, Zn, Bi,Mo, Ru, Ir. In one implementation the Ni(OH)₂ is deposited asclusters—for example having a height of about 0.7 nm and a width ofabout 8 to about 10 nm—may be deposited. The preferred range for eachimplementation is dependent on the substrate, geometry, orientation etc.Typically clusters of 1-2 monolayers thick are preferred due to theenhanced conductivity of these sized clusters. The size of the clustersneeds to be optimal to provide the activity without any stabilityissues. In one implementation the Ni(OH)₂ covers about 35% of theexposed electrode. The critical coverage needs to be determined as afunction of both the base metal and the hydr(oxy)oxide clusters. Asshown in FIG. 5A, there exists an optimal coverage for the HER. Therelative coverages of the Ni(OH)₂ clusters is expected to be differentbetween different elements, including elements within the same group(depending on oxophilicity, metal sites available etc. . . . ). It isbelieved there is little dependence on coverage at sufficient lowcurrent densities.

In one embodiment, specific electrolytes are utilized to provideadditional improvement to the reaction kinetics. Also, not consideringthe role of electrolyte components such as the cations have also limitedthe extent of improvement achievable. In one implementation, theHad-generation step can be further enhanced via alkali-metal induceddestabilization of the HO—H bond. For Li+, a 10-fold total increase inactivity has been observed. The electrolyte containing cations whichinteract strongly with OH species can be helped to influence thekinetics. This would include and not limited to Ba, Ca, Mg, and maybe insome cases Al, Be. The main criteria for working is that these ions arestable and do not undergo any formation of oxide or deposition duringthe recation as that would poison the catalyst. As for theconcentration, a whole range can be used from 0.001M to 1M for theelectrolyte.

It should be appreciated that the development of active sites for waterdissociation on well-defined single crystal surfaces can be fullytranslated to nanoscale systems. Further, systems may be synthesized bydeposition of metal oxides including, but are not limited to, Co, Fe,Mn, Pb, Zn, Bi, Mo, Ru, Ir. and or a mixture of these oxides on aplatinum or other noble metals such as Au, Ru, Ir etc. The metals rangefrom 3d metals such as Ni, Co, Cu, Mn, Fe, Cr, as well as V, W, Ti, Pd.This approach is applicable for relatively all metals that are stable.

Activity for HER and OER in an Alkaline

Activity for HER and OER in an Alkaline electrolytes can be enhanced byusing such an approach. The HER by using the advantage of bi-functionalcatalyst, and for the OER using the conductive hydr(oxy)oxides depositedon metal substrates. The metal substrate can be made from any metalwhich is relatively cheap and stable under the OER conditions.

In one embodiment, a stable metal substrate is made from a non-noble(cheap) metal core with a surface layer or two of the more stable metalin the OER region. A simple example will be using Fe/Cu core Au shellparticle which can then be modified with hydr(oxy)oxide clusters toobtain the conductive/stable oxides for the HER/OER. For the case of theHER, the choice of the surface layer metal can be either Pt, Ir or Niand for the OER the substrate could be Au, Ru, Ir etc.

Activity for HER and OER in an alkaline environment has not previouslybeen characterized. Characterization was carried out using thewell-characterized M^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) catalyst surfaces (M DNi, Co, Fe, Mn) to establish clear trends in activity for the HER andthe OER of a complex oxide system. Here, using 3d-M hydr(oxy)oxides,with distinct stoichiometries and morphologies in the hydrogen evolutionreaction (HER) and the oxygen evolution reaction (OER) regions, theoverall catalytic activities for these reaction are established as afunction of a more fundamental property, a descriptor, OH-M2C_ bondstrength (0≦≦1:5). This relationship exhibits trends in reactivity(Mn<Fe<Co<Ni), which is governed by the strength of the OH-M₂C_energetic (Ni<Co<Fe<Mn). These trends are found to be independent of thesource of the OH, either the supporting electrolyte (for the OER) or thewater dissociation product (for the HER). Using the OH_(ad)-M^(2+δ)interaction as the primary descriptor, it was observed that the activityfor the HER (bi-functional) and the OER (monofunctional) for these 3d-Mhydr(oxy)oxide systems follows the order Ni>Co>Fe>Mn. The increasing OERactivities for the 3d-M systems (always greater than Pt), as a functionof the decreasing strength of the OH_(ad)-M^(2+δ) interaction can beused to tune transition metal oxide catalysts for the OER. Thesuccessful identification of these electrocatalytic trends provides thefoundation for rational design of ‘active sites’ for practical alkalineHER and OER electrocatalysts.

Electrode preparation: extended surface electrode preparation. Pt(111)and Au(111) electrodes were prepared by inductive heating for 15 min at˜1,100 K (1,000 K for Au) in an argon hydrogen flow (3% hydrogen)35.Although preparation by inductive heating leads to formation of defectssuch as ad-islands on the surface, the number of such defects is lowenough to be electrochemically invisible. The annealed specimens werecooled slowly to room temperature under an inert atmosphere andimmediately covered with a droplet of deionized (DI) water. Electrodeswere then assembled into a rotating disk electrode (RDE) ensemble.Voltammograms were recorded in argon-saturated electrolytes. The Ag/AgClreference electrode was used, but all potentials in the paper are shownversus the reversible hydrogen electrode (RHE). Nanoparticles eithersynthesized in house or commercially acquired are then suspended inwater at a concentration of 1 mg/ml or higher and then deposited onto asubstrate such as glassy carbon and dried under Ar/H2 atmosphere. Theloadings are chosen to range from 50 micrograms/cm2 to 1 mg/cm2depending on the nature of materials.

Metal hydroxide deposition. All substrates considered in this work(freshly prepared extended surfaces, ad-island covered surfaces, as wellas high-surface-area catalyst electrodes) were washed thoroughly andintroduced into an electrolyte containing various concentrations oftransition metal perchlorates/chlorides. Concentration ranges testedinclude 150_(—)1,000 ppm. Following the introduction of the electrode,the oxide layers were then deposited either potentiostatically (held atpotentials above 0.6 V) or by cycling between H_(upd) and the OH_(ad)regions. Typical coverages of 30-40% were obtained within 10 min oftreatment. For higher coverages, higher concentrations as well as longertimes were used. After deposition, the electrodes were rinsed andintroduced into the clean electrochemical cell. The H_(upd) of themodified surface is compared against that of the bare surface toestimate the effective surface coverage of the oxide species.

Chemicals. All alkali metal hydroxides and perchlorate salts used wereobtained in the highest purity from Sigma Aldrich. Electrolytes used forour experiments, 0.1M KOH/LiOH, were prepared with Millipore DI water.All gases (argon, oxygen, hydrogen) were of 5N5 quality purchased fromAirgas. A typical three-electrode fluoro ethylene propylene (FEP)polymer based cell was used to avoid contamination from glasscomponents. Experiments were controlled using an Autolab PGSTAT 302Npotentiostat. Gold or platinum wires were used as counter electrodes forstudying hydrogen evolution reaction. Precautions were taken to preventsignificant accumulation of dissolved counter electrode ions near theworking electrode.

Electrochemical measurements. After extensive rinsing, the electrode wasembedded into the RDE and transferred into a standard three-compartmentelectrochemical cell containing 0.1M KOH/LiOH (Sigma-Aldrich). In eachexperiment, the electrode was immersed at 0.05 V in a solution saturatedwith argon. After obtaining a stable cycle between 0.05 and 0.7 V theelectrolyte was saturated with H₂, following which polarization curvesfor the HER were recorded on the disc electrodes between 0.05 V and −0.4V. The lower potential limits were chosen so as to avoid significantbubble formation, as well as to minimize the extended dissolution of thecounter electrode. The concentration of Pt after 1 h of HER measurementswas found to be less than 1 ppm in the working electrode compartment.All polarization curves were corrected for the infrared contributionwithin the cell. OER measurements were carried out by cycling theelectrode up to 1.7 V versus RHE. Potential hold experiments were alsocarried out for the oxide/Pt(111) systems to study the stability of theoxide clusters at these potentials, as well as for preparation ofsamples for the STM measurements.

XAS/XANES measurements. The X-ray absorption spectroscopy (XAS) datawere acquired at bending magnet beamline 12-BM-B at the Advanced PhotonSource (APS), Argonne National Laboratory. The synchrotron radiation wasfiltered by a double crystal Si(111) monochromator with a double mirrorsystem for focusing and harmonic rejection. A custom-made in situtransmission electrochemical X-ray cell with a 6 mm diameter Pt(111)single crystal and Ag/AgCl reference electrode was used in a grazingincidence geometry. A 13-element Ge detector (CANBERRA) was used tomeasure the fluorescence yield. Z-1 filters and grazing incidencegeometry was used to minimize the elastic scattering intensity. Themonochrometer calibration was monitored by simultaneously measuring thesame element reference foil in front of a Si diode and looking at theair-scattered beam. ±

X-ray absorption spectroscopy (XAS) data was collected and used tocharacterize the potential-dependent stoichiometry and structure of the3d-M hydr(oxy)oxides. FIG. 1 a summarizes the XAS results for theCo-oxide/Pt(111) system, as a representative of the 3d-M elementsconsidered in this work. Comparisons of reference samples at thesepotentials are also shown. The comparison reveals that δ=0 at −0.1 V andδ=1 at 1.4V. FIG. 1A illustrates sample XAS spectra forCo^(2+δ)O^(δ)(OH)_(2-δ) on Pt(111) surface for two different potentialsE=−0.1 V and E=+1.4 V. FIG. 1 b is a STM for Co(OH)₂/Pt(111) in the HERregion. Polarization curves for the HER for this surface are also shown(50 mVs⁻¹). The characteristic height of the clusters shown is ˜5.8 Åwith a diameter for 15-22 nm. FIG. 1C is a STM for CoOOH/Pt(111) andincludes the polarization curves for the OER. The characteristic heightof the clusters shown is ˜5.6 Å with a diameter for 15-22 nm. PurePt(111) polarization curves are shown for comparison in both FIGS. 1 band c. FIG. 1D is a comparison of OH_(ad) charge as a function ofoxophillicity of the metal oxide cation (M) for same coverages of theM^(2+δ)O^(δ)(OH)₂₋₈/Pt(111) surface. Enhanced adsorption of OH_(ad) isobserved as the larger area under he anodic peak at 0.6 V as well as theearly onset of the OH_(ad) butterfly region.

Results obtained from the XAS analyses for the other 3d-Mhydr(oxy)oxides are summarized in Table 1. With respect to Table 1, Mn⁺is the valence state of the 3d element; N is the first shellco-ordination number; M-O (Å) is the characteristic bond distancebetween the 3d metal centre and the oxygen anion. Depending on thenature of the 3d metal, different step changes in oxidation states areobserved in different potential regions. The rate of change of oxidationstates with potential are found to be dependent on the nature of theelements. In the HER, with the exception of Fe, all elements are in the+2 state. On the other hand, in the OER region, with the exception ofNi, all elements are in an oxidation state>+3. Fe, with its complexredox chemistry at these potentials, is known to exist in multipleforms, such as Fe(II) and Fe(III) oxides and hydroxides, and thereforeexhibits a valence state between 2.5 and 3.0. Nickel, exhibiting loweroxidation state at this potential, is not surprising, because for theNi-modified surfaces we found that, at 1.4V, the phase of Ni(OH)₂present on the Pt(111) surface resembles that of β-Ni(OH)₂ and thisphase of nickel hydroxide is known to remain stable up to highpotentials (<1:5 V; ref. 48) before undergoing complete transformationto NiOOH. The trend of oxidation state in the OER region presented hereagrees well with the established oxophilicity trends namely:Ni<Co<Fe<Mn.

TABLE 1 XAS Analysis for M^(2+δ) 0^(δ) (OH)_(2−δ) systems. Element HERPotential OER Potential Region (E < 0.0 V) Region (E ~ 1.4 V) ElementM^(n+) N M-0(Å) M^(n+) N M-0(Å) MN 2.0 ± 0.1 1.6 ± 0.3 2.10 ± 0.04 3.5 ±0.1 3.3 ± 0.4 1.89 ± 0.03 Fe 2.7 ± 0.1 2.5 ± 0.5 1.93 ± 0.03 3.0 ± 0.13.4 ± 0.3 1.95 ± 0.02 Co 2.2 ± 0.1 1.8 ± 0.4 2.02 ± 0.05 2.9 ± 0.2 3.9 ±0.5 1.89 ± 0.02 Ni 2.1 0.1 2.5 ± 0.4 2.06 ± 0.3  2.3 ± 0.1 3.2 ± 0.62.02 ± 0.03

Examination of X-ray absorption near edge structure (XANES) resultsreveals that, at ˜0.1 V, the Co(OH)₂ elements remain mostly in thevalence state+2 (Co²⁺), in the form of Co(OH)₂, whereas at 1.4 V thevalence state is close to +3 (Co³⁺), resembling CoOOH. Also included inTable 1 are the M-O distances and coordination numbers of thecorresponding 3d-M hydr(oxy)oxides, which, as expected, are dependent onboth the nature of the 3d elements as well as the applied electrodepotential. Overall examination of the XAS results, summarized in Table1, reveals several significant features. All 3d elements are found to bein valence states different from M(0), indicative of the absence ofmetal deposition on the Pt(111) substrates; the oxidation state of the3d-M elements is found to be dependent on potential, with the rate ofthis change being slowest for Ni and fastest for Mn. With the exceptionof Fe (ref. 27), the nature/stoichiometry of the 3d-M hydr(oxy)oxides at−0.1 V is found to be in the form of M(u) hydroxides. In contrast, at1.4 V, the nature/stoichiometry for these 3d-M hydr(oxy)oxides is foundto be more diverse, where Co and Fe are present in the M(iii)oxyhydroxide form, with Mn showing a higher valence state (˜3.5) with anunclear stoichiometry and Ni exhibiting the lowest valence state of˜2.3. X-ray absorption spectroscopy results clearly indicate that thesurface chemistry of the 3d oxides studied in this work is complex. Ageneral notation is used for simplicity, given by:M^(2+δ)O^(δ)(OH)_(2-δ) (0<δ<1.5), where δ=0 refers to the M(OH)₂ state(for example Ni, Co and Mn in the HER region) and δ=1 denotes the MOOHstate (for example Co, Fe in the OER region).

The morphology of the 3d-M hydr(oxy)oxide/Pt(111) surfaces was studiedusing scanning tunneling microscopy (STM). Again, the STM data for theCo(OH)2 and CoOOH clusters on Pt(111) are presented as a typical case(FIG. 1 b,c) similar observations were made for Fe and Ni species. TheSTM image in FIG. 1 b clearly shows that Co(OH)₂ particles are presentas clusters (with an approximate surface coverage of ˜40% for the imageshown) which are randomly distributed across the (111) terraces. TheCo(OH)₂ clusters have spheroid-like shapes, with characteristicdiameters of ˜7-10 nm and heights of ˜0.49-0.65 nm; the latter dimensioncorresponds to approximately two layers of electrically conductiveCo(OH)₂. The distribution of the clusters over the entire surfaceindicates that the clusters grow in a three dimensional (3D)(Volmer-Weber) fashion, seldom achieving a complete monolayer coverage.Importantly, after recording 50 cyclic voltammograms (CVs) between˜0.3V±0.4 V, the STM images of all 3d-M hydr(oxy)oxide/Pt(111) systemsremain the same, indicating that in this potential range the morphologyof the surface is stable. In contrast, for an electrode held at 1.55 Vthe STM image in FIG. 1 c indicates significant sintering of the CoOOHnanoparticles; a distribution of sizes ranging from ˜15 to 25 nm, withapproximately constant heights of two layers. This morphology was foundto be stable in the OER region (potential and time-independent). Giventhat similar morphological changes were exhibited by the otherM^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) systems, it is believed that thesewell-defined surface structures form the basis for any predictiveability in tailoring catalysts to have desirable reactivity for the HERand the OER in alkaline environments.

Electrochemical characterization was carried out by comparing the CVs ofthe Pt(111) and Co(OH)₂/Pt(111) surfaces (inset in FIG. 1 b). As was thecase for the XAS and STM analyses, the CV behavior of Co(OH)₂ is a fairrepresentation of the other 3d-M hydr(oxy)oxide systems (shown in insetof FIG. 1 d). Consistent with earlier reports for Pt(111), cyclicvoltammetry of Pt(111) (FIG. 1 b) shows that the adsorption ofunderpotentially deposited hydrogen (defined as the state of hydrogenadsorbed at a potential that is positive of the Nernst potential for thehydrogen reaction, H_(upd)) between 0.05 and 0.35 V is followed first bya wide double-layer potential region and then by the formation of anOH_(ad) adlayer (usually termed as the ‘butterfly’ region) between 0.6and 0.95 V. FIG. 2 (bottom inset) shows that the surface coverage of theH_(upd) (Θ_(Hupd)) is reduced by ˜45% on a Co(OH)₂/Pt(111) electrode.Other 3d-M hydr(oxy)oxide covered surfaces behave in a similar manner(inset FIG. 1 d), suggesting they act as a third body, selectivelyblocking the adsorption of H_(upd) without affecting the Pt—H_(upd)energetics. Thus, determination of the Θ_(Hupd) from CVs of modifiedPt(111) surfaces enables accurate determination of surface coverage bythe 3d-M hydr(oxy)oxide clusters. In the following, Pt(111) is alwaysmodified with nearly identical (˜45%) amounts of these clusters (FIG. 1d), thereby enabling the use of a well-characterizedM^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) electrode (0≦δ≦1.5).

In contrast to the Pt—H_(upd) bonding, FIG. 1 b shows that the effectsof a Co^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111)-modified Pt(111) surface onadsorption of OH— are significant. Three distinct features arenoteworthy: (1) the increase in the charge under the OH_(ad) (Θ_(OHad))region between 0.6 V<E<0.95 V, (2) negative shifts for both the onset ofthe OH⁻ adsorption and its desorption, and (3) OH_(ad)adsorption/desorption features are strongly irreversible. The magnitudeof the changes in the OH_(ad) charge is greater than what can occur onan unmodified Pt(111) surface; so it is believed that the oxideclusters' interaction with the OH provides an important contribution tothe observed value of Θ_(OHad). Similar behavior is exhibited by theother 3d elements considered here (FIG. 1 d); however, the chargecorresponding to the formation of OH_(ad) is strongly dependent on thenature of the 3d element and they follow the order Ni<Co<Fe<Mn. Thisresult clearly demonstrates that the oxophilicity of the 3d elements isstrongly dependent on their ‘nobility’ (position in the periodic table).This, in turn, follows the oxophilicity of the 3d-M hydr(oxy)oxides, aconclusion that is supported by density functional theory (DFT)calculations, specifically for Co and Mn oxide clusters. The DFTanalyses show stronger adsorption of OH on model two Co(OH)₂/Pt(111)films (treated as a film with two layers of Co(OH)₂ on the Pt(111)surface) than for clean Pt(111). Furthermore, the variation in bindingenergy moving from Co-based to Mn-based systems is consistent with theobserved oxophilicity derived from the Θ_(OHad) values.

The electrodes showing a linear variation in the Θ_(OHad) with thenature of the 3d-M elements (FIG. 1 d). The trend in the values ofΘ_(OHad) as a function of the 3d-M cation indicates that this quantitycorrelates well with the OH_(ad)-M^(2+δ) bonding. Although the physicalprocesses that are associated with the formation of OH_(ad) on metal andmetal oxide surfaces are not well defined, there is some consensus thatin alkaline solution the pseudocapacitance observed in the CVs ofPt(111) in the range 0.6<E<0.95 is just reversible OH⁻ adsorption on the(111) terrace sites. OH_(ad) species that are present at potentialsbelow 0.6 V, are found exclusively on the defect sites. The relativelysmall number of such defects on Pt(111) makes them invisible in the CVowing to the larger pseudocapacitive contributions from the H_(upd). Toovercome this limitation of the CV method, it is customary to ‘probe’the OH_(ad) with electrochemical reactions for which the OH_(ad) is areactant. The CO oxidation reaction was used to verify/validate theoxophilicity trends observed in the butterfly region at potentials below0.6 V.

The reaction mechanism for CO oxidation reaction has been wellestablished as following the Langmuir-Hinshelwood (L-H) pathway for Ptbimetallic systems such as PtSn and PtMo (refs 30,37). These catalystsare bi-functional in nature, where the CO adsorbs exclusively on the Ptsites and the OH_(ad) groups are present exclusively on the moreoxophilic Sn/Mo sites that facilitate the oxidative removal of CO. Inline with these systems, for the case of Pt(111) modified byM^(2+δ)O^(δ)(OH)_(2-δ) clusters, it believed that whereas CO is adsorbedexclusively on the Pt sites (CO_(bulk)

Pt—CO_(ad)), the OH_(ad) species adsorb preferentially on 3d-Mhydr(oxy)oxides (OH⁻+M^(2+δ)O^(δ)(OH)_(2-δ)

OH_(ad)-M^(2+δ)O^(δ)(OH)_(2-δ)+e⁻). The presence of OH_(ad) can then betested simply by monitoring the rate of CO oxidation at the constantelectrode potential through a L-H type reaction(Pt—COad+OH_(ad)-M^(2+δ)O^(δ)(OH)_(2-δ)+2OH⁻

HCO₃ ⁻+e⁻+H₂O). FIG. 2 illustrates polarization curves for CO oxidationon Co^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) and bare Pt(111) as typical examples;the corresponding activities for other 3d-element-modified electrodesare summarized in FIG. 2.

FIG. 2 is a chart of the elements are arranged in the order of theiroxophillicity from Mn to Ni. Pt is shown in the figure as a reference.Top inset: a comparison of the polarization curves for Pt (111) andPt(111) with 40% Cohydr(oxy)oxides for the CO oxidation reaction. As canclearly be seen, the onset potentials for CO oxidation are shifted ˜300mV negative from those of the bare Pt (111) surface. Bottom inset: aschematic showing the L-H mechanism for the CO oxidation reaction. COfrom bulk is found to adsorb on the free Pt site near the oxideclusters. OH_(ad) is formed by either adsorption of OH⁻ from theelectrolyte and/or a change in oxidation state of the cluster cationM^(2+δ). In the presence of CO_(ad) and OH_(ad) in each others vicinity,reaction between CO_(ad) and OH_(ad) species the occurs forming anintermediate which is eventually converted to (bi)-carbonates. The freeenergy for Pt—CO_(ad) is fixed, which enables the treatment of thesebi-functional metal-oxide/metal catalysts as a ‘pseudo’ mono-functionalcatalyst with a singular descriptor OH_(ad)-M^(2+δ).

Clearly, the CO oxidation current on Co^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) isshifted negatively by about 0.3 V with respect to the bare Pt(111)surface, indicating that this surface behaves as a bi-functionalcatalyst. As depicted schematically in FIG. 2 (bottom inset), thereaction proceeds along the perimeters of CO_(ad) islands andneighboring M^(2+δ) sites. The fact that the onset potential for the COoxidation reaction on Co-modified Pt(111) is observed at 0.1 V, stronglysuggests that oxygenated species must be present on theCo^(2+δ)O^(δ)(OH)_(2-δ) defect sites at these potentials. Although thesame conclusion commonly holds true for all otherM^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) systems, the reactivity of these surfacesfor the CO oxidation reaction is found to be dependent on the nature ofthe 3d element.

In general, the kinetics of the CO oxidation reaction on these surfacesis expected to be a function of both the Pt—CO_(ad) and OH_(ad)-M^(2+δ)energetics. If the Pt—CO_(ad) interaction is treated as independent ofthe nature of the 3d-M hydr(oxy)oxide, then the rate of the CO oxidationreaction should depend only on the OH_(ad)-M^(2+δ) energetics. Thus, byfixing the Pt—CO_(ad) energetics, it is possible to treat the reactionas a ‘pseudo’ mono-functional reaction that is controlled by thedescriptor related to OH_(ad)-M^(2+δ) bond strength. Indeed, FIG. 2reveals that the rate of the CO oxidation reaction is inverselyproportional to the OH_(ad)-M^(2+δ) bond strength, that is, the activityincreases in the order: Mn<Fe<Co<Ni. It is believed that, for a stronginteraction, such as that OH_(ad)-M^(2+δ), the CO oxidation reaction isinhibited because of the relatively low reactivity of OH_(ad). In fact,the OH_(ad)-M^(2+δ)_ bond is so strong that even pure Pt is more activethan the M^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111). In contrast, for Nihydr(oxy)oxides that bind OHad neither too strongly nor too weakly, weobserve the maximum activity for the CO bulk oxidation. The similarityin the trends observed for the activity for the CO oxidation reactionand the OH_(ad)-M^(2+δ) interaction strength in the butterfly region,confirms that the same guiding principle, namely the oxophilicity of the3d-M cation, is valid in the Hupd region as well. This indicates thatthe oxophilicity trends derived in FIGS. 1 d and 2 are valid between0.05 and 0.95 V.

As a starting point, the kinetic rates of the OER on the Pt(111)-oxideelectrode (dubbed hereafter as PtO) and the CoOOH/PtO electrode (shownas inset in FIG. 1 c). As were compared before, the Co system isrepresentative of the other M^(2+δ)O^(δ)(OH)_(2-δ) systems. FIG. 1 cshows that the onset of the OER on the CoOOH/PtO electrode is shifted by˜0.25 V, to more negative potentials, compared with PtO. Thesedifferences may reflect variations in the energetics (activationenergies and/or the enthalpies of adsorption) for the formation ofactive intermediates at these two surfaces, the exact values of whichare unknown. Nevertheless, the low activity of PtO, consistent withearlier reports, is indicative of the weaker OH_(ad)—PtO interaction,and thus it seems that the rate-determining step for such noble metalcatalysts could be the formation of OHad-PtO intermediates (OH⁻+PtO

OH_(ad)—PtO+e⁻). Along the same lines, the significant activityenhancement of the OER on the CoOOH/PtO electrode could be due to theenhanced interaction of OH— with CoOOH. To verify this, we have alsocompared the OER on the CoOOH/AuO electrode. The fact that the rate ofreaction on both these surfaces is the same (top inset of FIG. 3) isconfirmation that the OER reaction rate is controlled only byinteraction of reactants and reaction intermediates with CoOOH and notthe metal substrate. Given that the same is also true for the otherM^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) electrodes, these catalysts are purelymono-functional, a fact which is further confirmed by the monotonicvariation in OER activities as a function of ‘loading’ of the CoOOHclusters. As summarized in FIG. 3, the OER on M^(2+δ)O^(δ)(OH)_(2-δ)/PtOexhibits activities increasing from Mn to Ni hydr(oxy)oxides. FIG. 3 isa chart illustrating the trend in overpotential for the oxygen evolutionreaction (OER) is shown as a function of the 3d transition elements. Theelements are arranged in the order of their oxophilicity from Mn to Ni.Pt is shown in the figure as a reference. Top inset: a comparison ofpolarization curves for Pt(111) and Au(111) with 40% CoOOH for the OER.As can clearly be seen, the two potential curves are identical,suggesting a limited or no role played by the noble metal substrate forthis reaction. As a result, this reaction is classified as amono-functional reaction, and the main descriptor (as can be clearlyseen from the trend) is still the OH_(ad)-M³⁺ interaction. Bottom inset:a schematic showing the OER. OH⁻ from the bulk is found to adsorb on thefree catalyst site on the oxide clusters. The adsorbed OH groups OH_(ad)react with other such groups to form a reaction intermediate(re-combination), which is then further oxidized to O₂ and H₂O.

This suggests that the overall reaction rates are driven by the strengthof the interaction between the two oxidic species, the recombinationstep being the rate determining(2OH_(ad)-M^(2+δ)O^(δ)(OH)_(2-δ)→products), rather than by the initialadsorption step(OH⁻+M^(2+δ)O^(δ)(OH)_(2-δ)OH_(ad)-M^(2+δ)O^(δ)(OH)_(2-δ)+e⁻), which, asshown in FIG. 1 d, exhibits the opposite trend compared with that inFIG. 3. More than one recombined species might be formed during OERchemistry on oxides, but those mechanistic details do not affect ourinterpretations of the OER trends. In particular, too strong aninteraction between 3d-M hydr(oxy)oxides and OH_(ad) can lead to anadverse effect, wherein the reaction intermediates are stabilized,leading to a lower turnover frequency (defined as the number of completereaction events per site per second). This leads to poisoning of thesurface and a concomitant decrease in OER activities, as shownschematically in FIG. 3 (bottom inset). Thus, for the 3d elementsconsidered here, Ni, with its optimal interaction strength with OH_(ad),satisfies the Sabatier principle for catalyst design. Considering thatthe reactivity trends observed for the OER (Ni>Co>Fe>Mn) match thatobserved for the CO reactivity, we can conclude that the OH_(ad)-M^(2+δ)interaction (oxophilicity) trends can be extended up to the OERpotential regions. The trends in the energetics between OH_(ad)-M^(2+δ)for OH⁻, produced as the water dissociation product, which are relevantin the hydrogen evolution potential region, are similar to that for theOH_(ad) formed from the supporting electrolyte between 0.05 and 2.0 V.

In a bi-functional effect, the edges of Ni(OH)₂ clusters promote thedissociation of water (H₂O

H+ OH⁻+e⁻). The dissociation step is then followed by H adsorption onthe nearby Pt surfaces (H

Pt—H_(ad)) and by adsorption of OH⁻ on Ni(OH)₂ (see bottom inset in FIG.7). The kinetics of the HER will depend both on the rate of H_(ad)recombination, which is optimized on the Pt substrate⁴³, and on the rateof desorption of OH_(ad) to accommodate the adsorption of H₂O on Ni(OH)₂clusters. The presence of a bi-functional mode of catalysis for the HERis also confirmed by the observed enhancement for the HER activities onCo(OH)₂/Pt(111) systems (See FIG. 5). Further confirmation of thebi-functional mechanism was achieved by observing a distinct maximum inthe activity versus coverage of Co(OH)₂ as well as by comparing theCo(OH)₂/Au(111) systems with their Pt(111) counterparts. Thus, theoverall rate of the HER may, in principle, be controlled by optimizingthe density and the nature of the sites required for dissociation ofwater on M^(2+δ)O^(δ)(OH)_(2-δ), as well as OH⁻M^(2+δ)O^(δ)(OH)_(2-δ)and metal-H_(ad) energetics.

Here, using M^(2+δ)O^(δ)(OH)_(2-δ)/Pt(111) surfaces, the descriptorrelated to the adsorption energetics of Pt—H_(ad) is fixed, which theHER to be treated as a ‘pseudo’ mono-functional reaction: controlled bythe descriptor related to OH_(ad)-M^(2+δ) bond strength. FIG. 4illustrates The elements are arranged in order of their oxophillicityfrom Mn to Ni. Pt is shown in the figure as a reference. In the topinset: a comparison of polarization curves for Pt(111) and Au(111) with40% Co(OH)₂ for the HER. As can clearly be seen, the Au(111)/oxidesurface is significantly less active than the Pt(111) counterpart. Thisessentially establishes the role played by the Pt—H_(upd) descriptor. Onfixing this interaction, by using Pt(111) as the main substrate, we havefocused on the ‘pseudo’ mono-functional reaction with the OH*_(ad)intermediate on the oxide cluster, along with forming H_(ad)intermediate formed on the Pt substrate. The H_(ad) groups re-combine toform H2. Depending on the OH_(ad)-M^(2+δ) strength, the OH*_(ad) iseither stabilized (for Mn²⁺, Fe^(2+δ)) or destabilized (Ni²⁺, Co²⁺) onthe oxide clusters, which is found to dictate the turnover frequenciesfor these catalysts.

As summarized in FIG. 4, a monotonic relationship exists between the HERactivity and the OH_(ad)-M^(2+δ) with the most active catalysts beingNi(OH)₂/Pt(111) and the least active Mn(OH)₂/Pt(111). On the basis ofthe observed catalytic trends, it is clear that a balance must be foundbetween the transition state energies for water dissociation and thefinal state energies of adsorbed OH_(ad)-M^(2+δ)O^(δ)(OH)_(2-δ).According to the standard Brønsted-Evans-Polanyitype principles⁴⁶, thisresults in lower activation barriers for water dissociation, while alsoresulting in poisoning of the sites required for re-adsorption of watermolecules. The net result is that the turnover frequency substantiallydecreases (below bare Pt activity) for the Fe and Mn hydr(oxy)oxides onwhich OH_(ad) is more strongly adsorbed. In essence, this suggests thatthe Fe and Mn behave purely as spectators, blocking the Pt active sitesfor transforming H₂O to H₂. The best combination among the 3d elementsconsidered here, is found for the Ni(OH)₂/Pt(111), which has the mostfavourable balance between facilitating water dissociation andpreventing ‘poisoning’ with OH_(ad) (water dissociation product),together with the optimal Pt—H_(ad) energetics. The activity trendsderived for the HER using a series of 3d-M cations with differentinteraction strength with OHad clearly establishes the presence of OHadin the HER region (E<0 V). Most probably, the active sites for OH_(ad)are the defects, which are known to be very active for waterdissociation. We emphasize that the exact nature of OH_(ad)(electrosorption valency and free energy of adsorption) on theM^(2+δ)O^(δ)(OH)_(2-δ) cluster is not known unambiguously. However, thefact that the reactivity trends for the HER (Ni>Co>Fe>Mn), on surfaceswith constant Pt—H_(ad) interaction, are identical to the trends inoxophilicity, established from the CO oxidation reaction, OH⁻ adsorptionin the butterfly region and the OER above 1.6 V, strongly validates theuse of OHad-OH_(ad)-M^(2+δ) interaction strength as the descriptorcontrolling the HER on these M^(2+δ)O^(δ)(OH)_(2-δ)/Pt(Au) systems.

Example 1 Platinum and Lithium Ion Electrolyte

Conductive ultrathin Ni(OH)₂ clusters (height 0.7 nm, width 8 to 10 nm)were grown on both pristine Pt single-crystal surfaces and Pt surfacesmodified by two-dimensional (2D) Pt ad-islands [Pt-islands/Pt(111)].Relative to the corresponding Pt single-crystal surfaces, the mostactive Ni(OH)₂/Pt islands/Pt(111) electrodes in KOH solutions are moreactive for the HER by a factor of ˜8 at an overpotential of −0.1 V. TheHER is further improved by the introduction of solvated Li⁺ ions intothe compact portion of the double layer, resulting in a factor of 10total increase in activity.

Platinum

The atomic structures of Pt(111) and Pt(111) modified byelectrochemically deposited Pt islands, referred as Pt-islands/Pt(111)have previously been studied and characterized. Pt-islands/Pt(111) havepreviously been observed to impact cyclic voltammetry (CV) traces.Consistent with the higher oxophilicity of low-coordinated Pt sites, theonset of OH adsorption starts at more negative potentials on the Ptisland-covered electrode than on pristine Pt(111), whereas the OH_(ad)peaks are less reversible on the former surface.

FIG. 6D shows that in alkaline solution the Pt-islands/Pt(111) surfaceis ˜5 to 6 times more active for the HER than the corresponding pristinePt(111) surface. The inset shows XANES spectra for Ni(OH)₂ on Pt(111)shown for three different potentials: HER (−0.1 V), H_(upd) (0.1 V), andnear OER (1.2 V). Also shown is the reference for Ni(OH)₂. No shift inthe edge energy in XANES spectra between HER and H_(upd) regions isobserved. FIG. 6D also shows that, in acid solution, the HER on thePt-islands/Pt(111) electrode is improved by only a factor of ˜1.5. Inturn, this strong pH effect indicates that the low-coordinated Pt atomsmay have a significant effect on the rate determining step (rds) of theHER in alkaline solutions. Because the major difference between thereaction pathways in alkaline and acid solutions is that in alkalinesolutions, the hydrogen is discharged from water instead of fromhydronium ions (H₃O+) (12-14), it is believed that the large promotingeffect of low-coordinated Pt atoms in alkaline solution is due to morefacile dissociative adsorption of water. In turn, this would beconsistent with the Volmer reaction being the rds for the HER inalkaline electrolytes. The role of edge-step sites in acceleratingdissociative adsorption of water on metal surfaces is well documented inultra-high vacuum (UHV) environments (28). For materials withnear-optimal M−Had energetics (such as Pt), surface reactivity for theHER can be further improved by tailoring the active sites for moreefficient dissociative adsorption of water molecules.

Pt(111) and Pt-island/Pt(111) surfaces were modified by depositing3d-transition metal oxides even more active for water dissociation thanPt defect sites. The 3-d transition metal oxide was deposited asNi-(hydr)oxide clusters. The local symmetry; the oxidation state of Niatoms; and the number and identities of and distances between, nearestneighbor atoms were determined by insitu X-ray absorption spectroscopy(XAS) measurements. For example, from the analysis of the X-rayabsorption near edge structure (XANES) and extended X-ray absorptionfine structure (EXAFS) of the XAS spectra (as shown in inset of FIG.6D), Ni—O and Ni—Ni bond distances were found of 2.05±0.0 IÅ and3.08±0.0 IÅ. It was also determined that Ni remains mostly in the +2valence state, even after multiple hours of holding the electrodepotential at −0.1 V. Furthermore, from the edge shift (defined as thehalf-height energy of the normalized XANES edge step), between −0.1 Vand +0.8 V the change in the oxidation state of Ni is less than 0.5.These results suggest that stable Ni(OH)₂ clusters are the predominanthydr(oxide) form on the Pt(111) and Pt-islands/Pt(111) surfaces,especially in the HER potential region. Because the octahedral symmetryof the α and/or β forms of Ni(OH)₂ prevents p-d hybridization, theprominent pre-edge from Is->3d transitions implies that the Ni(OH)₂species are rich in defects. It is believed that such defectsparticularly active for dissociative adsorption of water molecules.

The surface morphologies of Ni(OH)2/Pt(1 11) andNi(OH)2/Pt-islands/Pt(111) is probed by STM and in situ surface x-ray(SXS) crystal truncation rod measurements. Although atomic resolutioncould not be obtained, the STM image in FIG. 6C clearly shows that theNi(OH)₂ clusters are randomly distributed across the (111) terraces. AllNi(OH)₂ clusters exhibited hemisphere-like shapes, with characteristicdiameters of ˜8 to 10 nm and heights of ˜0.7 nm, the lattercorresponding to two layers of Ni(OH)₂. This result indicates that theoxide exhibits Volmer-Weber (VW) type growth whereby three dimensional(3D) clusters of Ni(OH)₂ grow even at the lowest coverages. VW growth,in turn, is possible if the heat of adsorption of Ni(OH)₂ on Pt is lowerthan the cohesive energy of Ni(OH)₂.

The surface coverage of Ni(OH)₂ on Pt(111) is estimated from the STMimage, by measuring the area covered by the particles on the Pt(111)substrate. Using such analysis, the cluster density was found to reach amaximum at a surface coverage of ˜35% for this system. The STM image inFIG. 7A was acquired after deposition of Ni(OH)₂ on a Pt(111) surfacemodified by ˜0.2 monolayer (ML) of Pt islands. Clusters of Ni(OH)₂ inthe STM image appear elliposoidal with particle sizes between 4 and 12nm. Formation of both 3D Ni(OH)₂ clusters (having a predominantlyellipsoidal shape) and oxide free terraces were observed. The clustershad approximately constant heights of ˜O.8 nm but diameters ranging from4 to 12 nm. The same STM image, however, revealed no visible presence of2D Pt islands, suggesting that Ni(OH)₂ preferentially nucleates on thePt surface defects and that most of Pt islands are covered by Ni(OH)₂.In certain implementations, the most preferred form is the optimal sizedistribution, with small enough Ni(OH)2 clusters that the islands helpin nucleating the small hydr(oxy)oxide clusters growth. Thenanoparticles, and nanomaterials typically have such defects on thesurface of the particles as a result the hydr(oxy)oxide clusters grownon them will have the optimal properties. Thus, in certain embodiments,the only property that needs to be tuned the is the activity of thematerials.

The Ni(OH)₂ clusters play an important function in the formation ofH_(upd) and OH_(ad) adlayers on Pt(111) and Pt(111)/Pt-islandselectrodes. Addition of Ni(OH)₂ on the surface of both Pt(111) as wellas on the Pt(111) surface covered with Pt-islands, show systematicdecrease in the coverage of H_(upd) by ˜35%. This suggests that theNi(OH)₂ clusters selectively block the Pt sites corresponding toH_(upd). Furthermore, the two sharp H_(upd) peaks characteristic ofhydrogen adsorption/desorption on the Pt(111) electrode modified by the2D Pt islands, are completely suppressed on the surface covered by the3D Ni(OH)₂ clusters (FIG. 7A). This is additional evidence consistentwith the STM results that defects serve as the nucleation centers forelectrodeposition of Ni(OH)₂. It is believed that Pt islands arepredominantly covered by Ni(OH)₂. In contrast to the H_(upd) potentialregion, an enhanced adsorption of OH_(ad), which is accompanied byirreversible reduction of OH_(ad) on the negative-going sweep, isobserved on both electrodes, arising from the higher oxophilicity of thesurface elements.

Although in the presence of Ni(OH)₂ clusters there are 35% fewer Ptsites available for the HER than on the bare Pt(111) substrate, theNi(OH)₂/Pt(111) electrode is ˜7 times more active for the HER than thecorresponding bare Pt(111) electrode (FIG. 6D). Moreover, FIG. 7B showsthat the activity is further enhanced (˜8 times relative to barePt(111)) on the Ni(OH)₂/Pt-island/Pt(111) surface; at 6 mA/cm2, thedifference in overpotential between the HER in alkaline and acidsolutions is reduced to only 100 mV. On both surfaces, Ni(OH)₂ promotesthe dissociation of water and thereby enhances the rate of formation ofH_(ad) intermediates on the metal surface. As schematically depicted inFIG. 8, it is believed that water adsorption requires concertedinteraction of O atoms with Ni(OH)₂ and H atoms with Pt at the boundarybetween Ni(OH)₂ and Pt domains. Water adsorption requires concertedinteraction of) atoms with Ni(OH)₂ (broken orange spikes) and H atomswith Pt (broken magenta spikes) at the boundary between Ni(OH)₂ and Ptdomains. The Ni(OH)₂-induced stabilization of hydrated cations (AC⁺)(broke dark blue spikes) likely occurs through noncovalent (van derWaals-type) interactions. Hydrated AC⁺ can further interact with watermolecules (broken yellow spikes), altering the orientation of water aswell as the nature and strength of interaction of the oxide with water.Water adsorption is then followed by water dissociation and hydrogenadsorption (H_(ad)) on the nearby vacant Pt sites. Finally, two H_(ad)atoms on the Pt surface recombine to form H₂ (H₂ desorption step) and OHdesorbs from the Ni(OH)₂ domains followed by adsorption of another watermolecule on the same site.

From a surface reactivity standpoint, fruitful kinetic synergy(bi-functionality) between Ni(OH)₂ and Pt appears to be the key tomaximizing the rate of the HER. In FIGS. 7 A-B, incremental improvementsin activities for the HER in 0.1M KOH from the unmodified Pt(111)surface are shown for the hierarchical materials [ad-islands, Ni(OH)₂,and their combination] as well as the double layer (addition of Li⁺cations). The activity for the unmodified Pt(111) surface in 0.1 M HClO₄is shown for reference. Dashed arrow shows the activity trend. As shownin FIG. 7B, this bi-functionality, in turn, brings the activity of theHER in alkaline solutions very close to the activity of Pt in acidsolutions. In order to verify this conclusion, we have also compared theHER on Au(111) and Ni(OH)₂/Au(111) in alkaline solution. The relativelyweak interaction between Au and H_(ad) offsets the benefit of theenhanced water dissociation at the Au/Ni(OH)₂ interface. As a result,the rate of the HER on the Ni(OH)₂/Au(111) surface is much lower than onthe Pt(111)/Ni(OH)₂ surface, though the Au with Ni(OH)2 exhibits a muchhigher HER activity than pure Au as well. This further emphasizes theimportance of choosing the right metal-oxide/metal pairs in optimizingthe kinetics of the HER. Generally, metal with Ni(OH)2 will have higheractivity than the intrinsic metal. Two types of choices exist for theHER catalyst: high activity, higher cost catalyst, preferably Ir metalwith Ni(OH)2 and the cost effective material with a higher loading,preferably Ni metal (and its different forms such as Raney and alloys)with Ni(OH)2 modification. As for the OER: Ni and Co hydr(oxy)oxides areboth suitable materials but improved by the described conductive oxideclusters by decorating them on a metal substrate (preferably somethingstable in the OER region) such as Au.

Lithium Cations with Platinum Catalysts

There have been several recent studies that unambiguously showed thatthe rate of electrochemical reactions on Pt in alkaline solutions iscontrolled by the presence of alkali-metal cations (AC⁺). However, theseeffects have been entirely restricted to the potential region of acritical OHad coverage (E>0.6V), the latter species serving to stabilizehydrated cations in the compact part of the double layer throughnon-covalent (Van der Waals type) interactions. This stabilization leadsto the formation of OH_(ad)-AC⁺(H₂O)_(x) complexes that can eitherdecrease the reactivity of Pt by blocking the active sites foradsorption of reactants such as O₂, H2 and CH₃OH or, as in the case ofthe CO oxidation reaction, improve the reactivity of Pt via enhancedadsorption of OH_(ad).

For these purposes, the effect of hydrated Li cations was probed mainlybecause, in alkaline environments, Li⁺ is known to interact with H₂O andOH_(ad) more strongly than K⁺. Li⁺ cations have no effect on the HER onPt(111) surfaces. However, the results in FIG. 7B revealed that the HERon Ni(OH)₂/Pt-islands/Pt(111) is enhanced almost by a factor of two inthe presence of Li⁺ cations. This increase in activity has substantiallynarrowed the gap between the rates of HER on Pt in acid and, using thedescribed Ni(OH)₂/Pt-islands/Pt(111), in alkaline solution. FIG. 7Bshows that, at 5 mA/cm², the difference in overpotential between acidand alkaline environments is narrowed to only 35 mV. The fact that theactivity of the HER is affected by the nature of alkali metal cationsstrongly suggests that Ni(OH)₂/Li⁺—OH—H complexes are present in thecompact portion of the double layer. The presence of this complex, byitself, does not explain the 2-fold increase in HER activity. Itbelieved that the probability of water dissociation is significantlyenhanced via possible L⁺-induced steric and/or electronic effects on theinterfacial water structure and reactivity, as shown schematically inFIG. 8. Thus, Ni(OH)₂ plays a dual role: in addition to assisting withwater dissociation, it also provides an anchor to hold the beneficialLi⁺ ions in the compact portion of the double layer.

Example 1B Pt(110)

The same guiding principles for accelerating the Volmer reaction step inalkaline solutions are equally applicable to Pt(110). The CV and STMdata for the Ni(OH)₂/Pt-islands/Pt(110), are shown in FIG. 9A. Thegeneral characteristics (both structural and electrochemical) aresimilar to what was observed for the corresponding Pt(111) systems. Thecurrent densities for the HER on Pt(110) and Pt-nano systems arepresented in the logarithmic Tafel form (FIGS. 9B and 9D). As expected,the systematic modification of Pt(110), first with Pt islands and thenwith Ni(OHh exhibits an HER activity trend (FIG. 9B) with the same orderas the driving force for dissociative adsorption of water molecules, asdiscussed above for the Pt(111) systems:Pt(110)<Ni(OH)₂/Pt(110)<<Ni(OH)₂/Pt-islands/Pt(110). FIG. 9B As shown inFIG. 9B, at 10 mA/cm², the overpotential for the HER onNi(OH)₂/Pt-islands/Pt(110) in the presence of L⁺ is reduced by ˜100 mVcompared to bare Pt(110); From FIG. 9D this surface exhibits activities˜30 mV less than the activities Pt(110) recorded in acid solutions. TheTafel slopes lie in the range of 100-130 mV/dec, further emphasizing therole of Volmer step as the rate determining step for the HER in alkalinemedia. These results, in turn, verify the broad applicability of such ahierarchical catalyst design to various Pt extended surfaces.

Example 1C Nanocrystals

Finally, to demonstrate the generality of the behavior exhibited by theextended single crystal surfaces, the hierarchical design approach wasapplied to real nanocatalysts. To verify the applicability of theapproach for real electrocatalysts, conventional Tanaka Kikinzoku Kogyo(TKK) catalysts (5 nm Pt catalyst, FIG. 9C) were studied. Qualitativelysimilar trends were observed for nanocatalysts irrespective of shape orsize variations, but the results for the carbon supported Pt catalystsare shown. Nanoparticles, by their very nature, generally have asignificant surface density of low coordinated Pt sites and,consequently, no attempts were made to deposit Pt-islands on thesenanoparticles. Materials for the electrode, such as Pt, which have lowcoordinated cites, including nanomaterials, allow for the preferentialformation of Ni(OH)₂ clusters. As for extended surfaces, fractionalcoverage of Ni(OH)₂ was estimated to be ˜15 to 20% based on thesuppression of H_(upd) on a Pt-nano surface covered by Ni(OH)₂ (FIG.9C). Furthermore, the presence of Ni(OH)₂ on the Pt nanoparticles wasconfirmed by significant promotion of the HER (FIG. 9D); in particular,at 10 mA/cm², the difference in overpotential between a Pt-nanoelectrode in acid solution and a Li⁺/Ni(OH)₂/Pt-nano interface inalkaline solution narrowed to 40 mV. The roughness factor for the systemconsidered here defined as the ratio of actual area and the geometricarea is ˜6.5. The order of magnitude change observed in the HERactivities ˜50 times, for roughness factor change from 1 to 6.5 suggeststhat there is a significant scope of improvement in the overallactivities by simply optimizing the surface area/volume ratios of theseelectrocatalysts.

Transition Metals

In one implementation catalysts are made from transition metals such asNi, CO, Mn and Fe rather than noble metals. Transition metals arecommercially less expensive and further lower the catalyst cost therebyaiding in the development of cost-effective efficient alkalineelectrolyzers. This is achieved by using 3d-M hydr(oxy)oxides, withdistinct stoichiometries and morphologies in the HER and the OERregions.

The limited conductivity of transition metal oxides often leads toundesired ohmic losses. In the past it has been countered with the useof very high loadings of catalysts, alloying the oxides as well as usingconductive supports. However, given the high potentials at which the OERtakes place, most support materials suffer from loss in conductivitythereby decreasing the overall reaction activity. This has sometimesbeen overcome with the use of more noble metal oxides such as that of Ruand Ir oxides. These oxides, while they provide high activities stillsuffer from undesired dissolution thereby limiting the applicability ofthese systems. The lack of clearly defined catalyst sites in conjunctionwith catalyst degradation over time has often posed significantchallenges to the deployment of such materials into cost-effectiveelectrolyzers. Currently RuIr oxides are used supported on a conductivetitania. However, the dissolution issues have not been completelyidentified/understood and/or mitigated. However, the use of the methodsof one embodiment to synthesize conductive oxides of the presentinvention offers a unique means to circumvent the use of noble metaloxides without suffering significant losses in activities. Also, itprovides clearly defined active sites which opens up new avenues fordesign of catalyst materials, given the ability to achieve fundamentalunderstanding regarding the catalytic mechanism. Such catalysts providenew flexibility where one can use an inert substrate such as Au or moreactive (moderately stable) substrate such as Ir. This helps to achievesignificant OER activities with lower overall catalyst costs. Also,design of core-shell materials with the more noble metal specieslocalized to the surfaces of the catalyst particles will further help inlowering the cost of these catalysts. Given that the active materialsare 3d transition metal oxides, the cost of these catalysts can then besignificantly lowered without significant compromise in performance atall levels.

Example 2 Non-Noble Metal Catalyst

It has been observed that the HER on a Ni electrode modified by Ni(OH)₂nanoclusters is ˜4 times higher than on bare Ni surfaces, therebyproviding a means to enhance the activity of cost-effective catalystsfor alkaline electrolyzers. The HER results for IB group metals (M=Cu,Ag, Au) as well for the Pt group metals (M=Ru, Ir, Pt) and transitionmetals (3d-TM=Ni, V, Ti) modified by Ni(OH)2 was tested.

As summarized in FIG. 10, variations of HER activities in acid andalkaline environments will simply be expressed as the measuredoverpotential at a constant current density. For clarity, results willbe clustered into three groups; IB group metals, Pt group metals and3d-TM.

Experimental Procedure

Chemicals: Alkaline solutions were prepared from respective alkali salts(Multipharm, Sigma Aldrich, JT Baker and Alfa Aesar), perchloric acidfrom concentrated HClO4 (Sigma Aldrich) and Milli-Q de-ionized (DI)water. Pt, Ru, Ir, Au, Ag, Cu, Ti metal electrodes were 4N purity and 6mm in diameter.

Pristine Electrodes Preparation:

The electrodes were prepared by radio frequency (RF) annealing at ˜1100OC (Pt, Ru, Ir), ˜800 OC (Au, Ag, Cu), in a 3% H2-Ar gas mixture for 7minutes. Ti was polished, and de-oxidized in strong acid prior totesting. The samples were transferred into the electrochemical cell withthe surface protected with a drop of DI water and immersed underpotential control at 0.05 V vs. reversible hydrogen electrode (RHE)

Oxide Modified Electrodes:

Ni(OH)₂ modified electrodes were prepared by chemical deposition,wherein, the pristine electrode samples were immersed and equilibratedin 0.01-0.1M NiCl₂ (Sigma Aldrich) solutions for 4-12 hrs, and washedthoroughly before being introduced into the electrolyte. The procedure,which was found to provide the desired oxide coverage, was used for allthe materials. The time for the final oxide coverage was achieved byobserving no change in both the HER activities. Typical coverages of30-40% were obtained based on the H_(upd) of the modified surface of Ptand Ir compared with that of the bare surfaces. Coverages on the surfaceof other elements are hard to detect electrochemically but preliminaryXPS measurements reveal coverages ranging between ˜30-50%. No attemptwas made to control the coverages due to the lack of well-definedgeometry or surface “chemistry”. The coverage can be controlled bymodifying the surface geometry (ad-islands), time of deposition,concentration of deposition, time of deposition and finally by changingthe nature of the metal substrate.

Electrochemical Measurements:

A typical three electrode FEP cell was used to avoid contamination fromglass components, for the RDE measurements. Ag/AgCl reference (−0.96 Vvs. RHE), were used for electrochemical measurements. The counterelectrodes used were Au (Au, Ag and Ti), Cu (for Cu) and Pt (Pt, Ru,Ir). The sweep rates used in the CV experiments were 50 mVsec-1, whilethe rotation rate was 1600 rpm. For the HOR and HER experiments thepotential was swept in the cathodic direction from the hold potential;the data presented is taken from first sweep curves. IR compensationswere applied for all the data reported here. Experiments were controlledusing an Autolab PGSTAT 302N potentiostat. The gases used were researchgrade (5N) Ar and H2. The HER measurements were also performed atdifferent scan rates (50 mV/s, 20 mV/s, 5 mV/s) with very little changein activity values reported. Also, rotating ring disc electrode (RRDE)measurements were performed for various systems to quantify the HERcurrents. For the ring electrode, with a collection efficiency of 21%,the currents from the ring and disc electrodes confirmed that all thecurrents measured were from the hydrogen evolution reaction and thecontributions from other side reactions were negligible.

Example 2 Results

In acid environments (FIG. 10), three distinct relationships arenoteworthy: (i) while there is no difference in the activities betweenPt and Ir (presumably due to similar metal-Had energetics), Ru is theleast active either due to strong Ru-Had interaction and/or the presenceof oxygenated species even in the HER potential region; (ii) for the IBgroup elements, the activity trends increase in order Au>Ag>Cu,signaling that the HER activities may increase in the same order as thehydrogen adsorption energy on the IB group metals; (iii) consideringthat in electrochemical environments all of the 3d-TM elements arecovered by oxides/hydroxides (with unknown stoichiometries) in varyingdegrees, the observed trends (Ni>V>Ti) may or may not correspond to theM−Had energetics. FIG. 10 inset illustrates 2-D representations of themechanism involved with transformation of protons (pH=1) and water(pH=13) into H₂. IB group exhibits opposite trends in acid vs. alkalinesolutions. Pt-group metals show no discernible difference in acidsolution between Ir and Pt, but a much larger potential difference inalkaline solution. These differences are found to disappear when wecompare the trends between acid and Ni(OH)₂/M surfaces in alkaline,confirming the role of water dissociation step. No clear understandingis possible for 3d transition metals due to the poorly defined surfaceproperties of such materials which are often covered with “oxide”species. However, given the HER activity is enhanced for all thesemetals with the introduction of Ni(OH)₂, it is clear that the nature ofthe oxide species is important for the water dissociation step relevantfor the HER.

Catalytic activity of the HER at high pH values can be simultaneouslycontrolled by the M−Had bond strength as well as by the energy requiredto dissociate water into H and OH⁻. From FIG. 10 it is believed that:(i) activities in alkaline are significantly lower than in acidsolutions, consistent with the observed inferior activities of the HERin alkaline solution; (ii) unlike in acidic media, in alkaline solutionsthe activity increases in the order Ir>Pt>Ru, suggesting that for asimilar energy of hydrogen adsorption on Pt and Ir, the rate of Volmerstep is enhanced on more oxophilic Ir; (iii) also, the activity trendfor the IB metals is inverse to one found in acids (Cu>Ag>Au), signalingthat the rate of reaction may be controlled by the dissociation energyof water rather than by the adsorption energy of hydrogen; and (iv) theorder of activity of the 3d-elements is the same as in acidicenvironments, confirming that the nature as well as the coverage bysurface oxides may be more important than the energy required for thewater dissociation step. Based on these observations and keeping in mindthat the M−Had binding energy should be pH-independent, it is believedthat the rate of the water dissociation step must provide an importantcontribution to the observed pH-variations in activity trends.Consequently, then, for the catalysts with comparable M−Had energetics,improving the water dissociation step can improve the alkaline HERactivities.

As noted above, the electrocatalytic trend for the HER has beenestablished on 3d-TM(OH)₂/Pt “pseudo” mono-functional catalysts withNi(OH)₂/Pt having the highest activity. Here, the bi-functionality ofNi(OH)₂/3d-TM can be transformed to a “pseudo” mono-functional type ofcatalysts, which is dependent on the substrate-Had interaction, akin tothe acid HER. As shown in FIG. 10, the Ni(OH)₂/M surfaces are alwaysmore active (˜3-5 fold) than on corresponding bare substrates. In turn,this suggests that, the edges of Ni(OH)₂ clusters do promote thedissociation of water and the generated hydrogen is “collected” andrecombined on the substrate sites at a rate similar to that in acidsolutions. On the basis of this, one should expect that reactivitytrends on Ni(OH)₂/M catalysts should be very similar to one observed inacidic solutions. Indeed, the trends for the HER on IB metalsestablished in acid solution are re-established in alkaline solutions onsurfaces modified by Ni(OH)₂ (Au>Cu>Ag), implying that the rates arecontrolled again by the M−Had energetics. Furthermore, there is nodiscernible difference in activity between Ni(OH)₂/Pt and Ni(OH)₂/Ir,indicating that, as in acid solution, the HER is almost completelycontrolled by a similar hydrogen adsorption energy on these twosurfaces.

Finally, the overpotentials for the HER on Ni(OH)₂/3d-TM issignificantly smaller, depending on the nature of 3d elements, rangingfrom 0.2 to 0.5 V. It is believed that Ni(OH)2 serves to enhance thewater dissociation step (it does do so on IB-group metals and Pt-groupmetals) and its ability is superior to that of the native oxide speciespresent on the 3d-TM surfaces. The presence of Ni(OH)₂ on these 3d-TMsystems was confirmed by analyzing X-ray absorption spectra (XAS) forthe Ni(OH)₂N system. To determine the presence of the oxide species onthe Ni electrode, ex situ X-ray photoelectron spectroscopy (XPS)analysis was performed. These results reveal two chemical states; themetallic state at 852 eV (2p^(3/2)) and 869 eV (2p^(1/2)) and the oxidestate characterized by the binding energy of 855 eV (2p^(3/2)). AlthoughXPS methods are not surface sensitive it may indicate that on the Nielectrode there are some metal sites that, in turn, may serve to“collect” and recombine the hydrogen atoms produced from the waterdissociation step on Ni(OH)2 through a similar bifunctional mechanism.

In certain embodiments, it is important to tailor the active sites forthe HER in alkaline solutions. Ag and Cu have unique advantages due tothe relatively low affinity toward formation of hydrides over long-termoperations, while Ni offers the highest activity. Importantly, theresults for the HER on Ag, Cu and Ni modified by Ni(OH)2 (FIG. 11)reveal that the activities of these surfaces nearly match those in acidsolutions on un-modified electrodes. Furthermore the activity ofNi(OH)2/Ni is enhanced by a factor of 4 vs. the Ni(OH)2-free Nisurfaces, confirming that the nature of oxide species on the surface iscritical. Thus, for some hydroxide modified “metal” surfaces the HER canbe made even more active in alkaline solutions than in acidenvironments. This is particularly relevant for Ni, Ni alloys and RaneyNi, which are used as commercial catalysts in alkaline environments.Extending the bi-functional approach to such systems will, in turn, helpfurther toward bridging the gap between noble and nonnoble HER “metal”catalysts, especially with the possibility of using higher loading ofnon-noble materials. For the HER in alkaline medium there is a synergybetween the effectiveness of the catalyst to break water molecules andto efficiently form hydrogen that subsequently can be adsorbed andassociated on metal surfaces.

The elements are arranged in the order of their oxophillicity from Mn toNi. Pt is shown in the figure as a reference. Top inset—a comparison ofthe polarization curves Pt(111) and Pt(111) with 40% Cohydr (oxy)oxidesfor the CO oxidation reaction. As can clearly be seen, the onseenpotentials CO oxidation are shifted ˜300 mV negative from those of thebare Pt (111) surface. Bottom inset: a schematic showing the L-Hmechanism for the CO oxidation reaction. CO from bulk is found to absorbon the free Pt site near the oxide clusters. OH_(ad) is formed by eitheradsorption of OH⁻ from the electrolyte and/or a change in oxidationstate of the cluster cation M^(2+δ). In the presence of CO_(ad) andOH_(ad) in each others vicinity, reaction between CO_(ad) and OH_(ad)species the occurs forming an intermediate which is eventually convertedto (bi)-carbonates. The free energy Pt—CO_(ad) is fixed, which enablesthe treatment of these bi-functional metal-oxide/metal catalysts as a‘pseudo’ mono-functional catalyst with a singular descriptorOH_(ad)-M^(2+δ)

The foregoing description of illustrative embodiments has been presentedfor purposes of illustration and of description. It is not intended tobe exhaustive or limiting with respect to the precise form disclosed,and modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. An electrode for use in the hydrogen evolutionreaction, comprising: an electrode metal; a plurality of metal(hydr)oxide clusters deposited on the surface of the electrode metal;wherein the electrode exhibits bifunctionality with respect to thehydrogen evolution reaction.
 2. The electrode of claim 1, wherein theelectrode metal comprises a plurality of low coordination areas on itssurface.
 3. The electrode of claim 2, wherein the low coordination areascomprise a plurality of surface defects.
 4. The electrode of claim 3,wherein substantially all of the plurality of metal (hydr)oxide clustersare deposited on the plurality of surface defects.
 5. The electrode ofclaim 1, wherein the electrode metal comprises a transition metal. 6.The electrode of claim 1, wherein the metal (hydr)oxide consistsessentially of Ni(OH)₂.
 7. The electrode of claim 1, about 35% of theelectrode is covered with deposited metal (hydr)oxide clusters.
 8. Theelectrode of claim 1, wherein the metal (hydr)oxide clusters have aheight of about 0.7 nm and a width of about 8 nm to about 10 nm.
 9. Aelectrolytic cell comprising: an anode; a cathode having a plurality ofmetal (hydr)oxide clusters deposited on the surface of the electrodemetal; an electrolyte; wherein the electrode exhibits bifunctionalitywith respect to the hydrogen evolution reaction.
 10. The electrolyticcell of claim 9, wherein the electrolyte is an alkaline electrolyte. 11.The electrolytic cell of claim 9, further comprising lithium ions. 12.The electrode of claim 9, wherein the electrode metal comprises aplurality of low coordination areas on its surface.
 13. The electrode ofclaim 12, wherein the low coordination areas comprise a plurality ofsurface defects.
 14. The electrode of claim 13, wherein substantiallyall of the plurality of metal (hydr)oxide clusters are deposited on theplurality of surface defects.
 15. The electrode of claim 9, wherein theelectrode metal comprises a transition metal.
 16. The electrode of claim9, wherein the metal (hydr)oxide consists essentially of Ni(OH)₂.
 17. Amethod of generating hydrogen comprising: forming a cell, the cellhaving a cathode, an anode, and an alkaline electrolyte, the cathodehaving a plurality of metal (hydr)oxide clusters deposited thereon;applying a current to the cell; facilitating disassociation of water andthe production of hydrogen intermediates at the plurality of metal(hydr)oxide clusters; adsorbing hydrogen intermediates to the cathodesurface; and combining hydrogen intermediates to form molecularhydrogen.
 18. The method of generating hydrogen of claim 17, wherein themetal (hydro)oxide clusters comprise Ni(OH)₂.
 19. The method ofgenerating hydrogen of claim 17, wherein the cathode comprises Ir. 20.The method of claim 17 wherein the cathode comprises Ni.